of: mdio: provide devm_of_mdiobus_register()

Implement a managed variant of of_mdiobus_register(). We need to make
mdio_devres into its own module because otherwise we'd hit circular
sumbol dependencies between phylib and of_mdio.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>